late 14c., "to employ, put into active use," from exercise (n.); originally "to make use of;" also in regard to mental and spiritual training. The sense of "engage in physical activity" is from 1650s. Also from late 14c. in the sense of "train, drill, discipline, educate (someone); develop (a skill) by practice." Related: Exercised; exercises; exercising.
mid-14c., "condition of being in active operation; practice for the sake of training," from Old French exercice (13c.) "exercise, execution of power; physical or spiritual exercise," from Latin exercitium "training, physical exercise" (of soldiers, horsemen, etc.); "play;" in Medieval Latin also of arts, from exercitare, frequentative of exercere "keep busy, keep at work, oversee, engage busily; train, exercise; practice, follow; carry into effect; disturb, disquiet," from ex "off" (see ex-) + arcere "keep away, prevent; contain, enclose," from PIE *ark- "to hold, contain, guard" (see arcane).
The original notion in the Latin verb is obscure. Meaning "physical activity for fitness, etc." first recorded in English late 14c. Also from late 14c. as "a carrying out of an action; a doing or practicing; a disciplinary task." In reference to written schoolwork from early 17c. The ending was abstracted for formations such as dancercise (1967); jazzercise (1977); and boxercise (1985).
circular piece of stone or metal plate about 12 inches in diameter, pitched from a fixed spot the greatest possible distance as a gymnastic exercise and an athletic contest, 1650s, from Latin discus "discus, disk," from Greek diskos "disk, quoit, platter," related to dikein "to throw," which is perhaps from PIE *dik-skos-, from root *deik- "to show, pronounce solemnly; also in derivatives referring to the directing of words or objects" [Watkins]; but Beekes says dikein is of Pre-Greek origin. The notion is "to throw" as "to direct an object."